Magnetoresistive Random Access Memory (MRAM), based on the integration of silicon CMOS with MTJ technology, is a major emerging technology that is highly competitive with existing semiconductor memories such as SRAM, DRAM, Flash, etc. A MRAM device is generally comprised of an array of parallel first conductive lines on a horizontal plane, an array of parallel second conductive lines on a second horizontal plane spaced above and formed in a direction perpendicular to the first conductive lines, and an MTJ element interposed between a first conductive line and a second conductive line at each crossover location. A first conductive line may be a word line while a second conductive line is a bit line or vice versa. Alternatively, a first conductive line may be a bottom electrode that is a sectioned line while a second conductive line is a bit line (or word line). There are typically other devices including transistors and diodes below the array of first conductive lines as well as peripheral circuits used to select certain MRAM cells within the MRAM array for read or write operations.
An MTJ element may be based on a tunneling magneto-resistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. In an MRAM device, the MTJ element is formed between a bottom electrode such as a first conductive line and a top electrode which is a second conductive line. An MTJ stack of layers that are subsequently patterned to form an MTJ element may be formed in a so-called bottom spin valve configuration by sequentially depositing a seed layer, an anti-ferromagnetic (AFM) pinning layer, a ferromagnetic “pinned” layer, a thin tunnel barrier layer, a ferromagnetic “free” layer, and a capping layer. The AFM layer holds the magnetic moment of the pinned layer in a fixed direction. In a MRAM MTJ, the free layer is preferably made of NiFe because of its reproducible and reliable switching characteristics as demonstrated by a low switching field (Hc) and switching field uniformity (σHc). Alternatively, an MTJ stack may have a top spin valve configuration in which a free layer is formed on a seed layer followed by sequentially forming a tunnel barrier layer, a pinned layer, AFM layer, and a capping layer.
The pinned layer has a magnetic moment that is fixed in the “y” direction, for example, by exchange coupling with the adjacent AFM layer that is also magnetized in the “y” direction. The free layer has a magnetic moment that is either parallel or anti-parallel to the magnetic moment in the pinned layer. The tunnel barrier layer is thin enough that a current through it can be established by quantum mechanical tunneling of conduction electrons. The magnetic moment of the free layer may change in response to external magnetic fields and it is the relative orientation of the magnetic moments between the free and pinned layers that determines the tunneling current and therefore the resistance of the tunneling junction. When a sense current is passed from the top electrode to the bottom electrode in a direction perpendicular to the MTJ layers, a lower resistance is detected when the magnetization directions of the free and pinned layers are in a parallel state (“1” memory state) and a higher resistance is noted when they are in an anti-parallel state or “0” memory state.
In a read operation, the information stored in an MRAM cell is read by sensing the magnetic state (resistance level) of the MTJ element through a sense current flowing top to bottom through the cell in a current perpendicular to plane (CPP) configuration. During a write operation, information is written to the MRAM cell by changing the magnetic state in the free layer to an appropriate one by generating external magnetic fields as a result of applying bit line and word line currents in two crossing conductive lines, either above or below the MTJ element. In certain MRAM architectures, the top electrode or the bottom electrode participates in both read and write operations.
A high performance MTJ element is characterized by a high magnetoresistive (MR) ratio which is dR/R where R is the minimum resistance of the MTJ element and dR is the change in resistance observed by changing the magnetic state of the free layer. A high MR ratio of over 30% and a low magnetostriction (λS) value of about 1×10E-06 or less are desirable. This result is accomplished by (a) well controlled magnetization and switching of the free layer, (b) well controlled magnetization of a pinned layer that has a large exchange field and high thermal stability and, (c) integrity of the tunnel barrier layer. In order to achieve good barrier properties such as a specific junction resistance×area (RA) value and a high breakdown voltage (Vb), it is necessary to have a uniform tunnel barrier layer which is free of pinholes that is promoted by a smooth and densely packed growth in the AFM and pinned layers. Although a high RA value of about 10000 ohm-μm2 is acceptable for a large area (A), RA should be relatively small (<1000 ohm-μm2) for smaller areas. Otherwise, R would be too high to match the resistivity of the transistor which is connected to the MTJ.
In addition to MRAM applications, an MTJ element with a thinner tunnel barrier layer to give a very low RA (<5 ohms-μm2) may be employed in TMR sensor head applications. Referring to FIG. 1, a portion of a TMR read head 20 on a substrate 21 is shown from the plane of an air bearing surface (ABS). There is an MTJ element 23 formed between a bottom lead 22 which is a bottom shield (S1) and a top lead 30 which is an upper shield (S2). The MTJ element 23 is comprised of a seed layer 24, an AFM layer 25, a pinned layer 26, a tunnel barrier layer 27, a free layer 28, and a cap layer 29 which are sequentially formed on the bottom lead 22 and have a composition and function similar to the corresponding layers in the MTJ element described previously. The free layer 28 may be a composite CoFe/NiFe layer. In this example, a NiFe layer in the bottom lead 22 represents S1 and a NiFe layer in the top lead 30 represents S2. A read operation involves moving the read head along the ABS in the z direction over a recording medium which causes an external magnetic field to influence the magnetization direction of the free layer.
Generally, the purpose of the capping layer is to protect underlying layers in the MTJ during etching and other process steps and to function as an electrical contact to an overlying conductive line. The typical capping layer for an MTJ stack is a non-magnetic conductive metal such as Ta or TaN. According to M. Nagamine et. al in “Conceptual material design for MTJ cap layer for high MR ratio” in abstract ED-10, 50th MMM conference, San Jose, Calif. (2005), a Ta capping layer yields a higher dR/R than a Ru capping layer. This result is due to a higher oxidation potential for Ta than for Ru. It is also known that NiFe with a Ru cap is positively charged while NiFe with a Ta cap is negatively charged. Thus, Ta is much more reactive with oxygen in the free layer and is a more efficient “getter” than Ru. As stated by W. Egelholf et. al in “Oxygen as a surfactant in the growth of giant magnetoresistive spin valve” in J. Appl. Phys., 82, p. 6142-51 (1997), oxygen is highly mobile in the transition metals and alloys thereof such as NiFe, CoFe, Cu, and Ru and has a strong tendency to float out to the surface. During thermal annealing, Ta is capable of gettering oxygen atoms originating in the NiFe free layer. Consequently, the NiFe free layer is less oxygen contaminated and a more distinct boundary between the tunnel barrier layer and NiFe free layer is thereby obtained to improve dR/R. The disadvantage of using a Ta capping layer is that Ta diffuses into NiFe during thermal annealing to produce an alloy that not only reduces free layer moment (Bs) but makes NiFe very magnetostrictive with a λS of ≧5×10−6. Thus, an improved capping layer is needed that prevents inter-diffusion between a free layer and capping layer, serves as a good oxygen getter material, and enables both a high MR ratio and low λS value to be achieved in MTJs for advanced MRAM and TMR read head technologies.
According to a search of the prior art, hafnium (Hf) has been used in various ways to influence the performance of magnetic devices. In U.S. Pat. No. 6,903,909, an amorphizing agent such as Hf is inserted in a NiFe pinned layer to form a NiFe/NiFeHf/NiFe configuration that smoothes the pinned layer and thereby reduces FM coupling between the pinned layer and free layer. U.S. Patent Application 2006/0114716 describes a non-magnetic hafnium layer that is inserted into a free layer to lower the switching magnetic field by weakening the exchange coupling between the two adjacent ferromagnetic layers. U.S. Patent Application 2006/0023492 discloses a MTJ with a low aspect ratio elliptical shape in which magnetic layers are doped with various elements like Hf to facilitate a flux closure configuration and a vortex magnetization state in the free layer and reference layer. In U.S. Patent Application 2002/0054462, a MTJ with an insulating barrier made of an oxidized thin metallic alloy of Ni and another non-magnetic material such as Hf is described that produces a barrier layer with a relatively low barrier height that allows low junction resistance and a high TMR ratio. U.S. Patent Application 2006/0056114 discloses a composite magnetic layer that may include Hf which is formed between a tunnel barrier and a pinned layer to prevent migration of Ni or Mn into the tunnel barrier.
Magnetic layers comprised of an alloy may be deposited by a sputtering technique. There are several references in the prior art where a magnetic layer is deposited in a sputtering system by co-sputtering two targets. In U.S. Pat. No. 6,893,714 and related U.S. Patent Application 2005/0271799, a ferromagnetic alloy and a non-magnetic oxide are co-sputtered to form a magnetic layer. U.S. Patent Application 2006/0002026 describes a reactive sputtering process where a magnetic recording material and a matrix material such as SiOX may be co-deposited on a substrate. U.S. Patent Application 2002/0045070 describes co-sputtering with a non-magnetic target (oxide) and a magnetic target to form fine magnetic dots dispersed in a non-magnetic matrix.